The operating temperature within a gas turbine is both thermally and chemically hostile. Significant advances in high temperature capabilities have been achieved through the development of iron, nickel, and cobalt-based superalloys and the use of environmental coatings capable of protecting superalloys from oxidation, hot corrosion, etc., but coating systems continue to be developed to improve the performance of the materials.
In the compressor portion of a gas turbine, atmospheric air is compressed to 10-25 times atmospheric pressure, and adiabatically heated to 800° F.-1250° F. (427° C.-677° C.) in the process. This heated and compressed air is directed into a combustor, where it is mixed with fuel. The fuel is ignited, and the combustion process heats the gases to very high temperatures, in excess of 3000° F. (1650° C.). These hot gases pass through the turbine, where airfoils fixed to rotating turbine disks extract energy to drive the fan and compressor of the turbine, and the exhaust system, where the gases provide sufficient thrust to propel the aircraft. To improve the efficiency of operation of the turbine, combustion temperatures have been raised. Of course, as the combustion temperature is raised, steps must be taken to prevent thermal degradation of the materials forming the flow path for these hot gases of combustion.
Using certain known alloys results in coarse grain size and grain boundary cracking under such conditions. For example, it is well know that coarse grain size and grain boundary cracking may be concerns for Inconel Alloy 706 components such as rotors and turbine disks (Inconel is a registered trademark of Inco Alloys International, Inc., Huntington, W. Va.). As identified in AMS specification 5703B, Inconel Alloy 706 has the composition by weight of carbon 0.06 max, manganese 0.35 max, phosphorus 0.35 max, sulfur 0.015 max, chromium 14.5 to 17.5, nickel 39 to 44, niobium 2.5 to 3.3, titanium 1.5 to 2, aluminum 0.4 max, boron 0.006 max, copper 0.3 max, and a balance of iron.
Known processes attempt to remedy this susceptibility to cracking by focusing on the forging process and the heat treatment processes. For example, two-step and three-step aging processes have been used to generate Eta phase along grain boundaries, which reduces the crack growth rate along the grain boundaries. However, the aging heat treatment is applied after uncontrolled grain growth already took place during forging and/or during solution heat treatment. As a result, the forgings typically have a very coarse grain size, which can increase intergranular cracking susceptibility.
Inconel Alloy 706 may also form grain boundary carbide films. Carbides having high chromium content can be easily dissolved at forging temperature. As a result, chromium redistributes along the grain boundaries as carbide films during the cooling. This may lead to embrittlement and significantly increased intergranular cracking susceptibility.
Known alloys add rhenium and change the aluminum-niobium ratio to reduce the coarsening rate of gamma double prime phase. However, these approaches have no impact on the grain coarsening and intergranular cracking. In other known alloys, chromium content is increased (for example, to about 18%) and titanium content is increased (for example, to about 1.9%). This creates a stronger alloy with reduced ductility.
A alloy and process of forming a alloy controlling grain size and grain boundary that does not suffer from the above drawbacks would be desirable in the art.